Retroreflective articles have the ability to return a substantial portion of incident light in the direction from which the light originated. This unique ability has led to widespread use of retroreflective articles on clothing. Persons who work or exercise near motor vehicle traffic need to be conspicuously visible so that they do not get struck by passing motor vehicles. Retroreflective articles serve the purpose of highlighting a person's presence by retroreflecting light from motor vehicle headlamps or other light sources.
A retroreflective article typically comprises a layer of optical elements, a polymeric binder layer, and a specular reflective layer. The optical elements usually are transparent microspheres that are partially embedded in the polymeric binder layer such that a substantial portion of each microsphere protrudes from the polymeric binder layer. Typically, the binder layers in such articles are elastomeric compositions of polyacrylates, polyolefins, phenolic cured resins, isocyanate polymers with active hydrogens such as two part urethanes or aminoplast- or amine-cured hydroxy functional polymers. The specular reflective layer typically comprises aluminum or silver, and this reflective layer is disposed beneath the embedded portion of the transparent microspheres. Light striking the front surface of the retroreflective article passes through the transparent microspheres, is reflected by the specular reflective layer, and is collimated by the transparent microspheres to travel back in the direction of the light source.
Retroreflective articles must be able to withstand laundering conditions when used on clothing. If the retroreflective article is not durable to laundering, and retroreflectivity becomes substantially diminished after repeated washings, the clothing cannot continue to serve its safety function by highlighting the wearer's presence. Investigators in the retroreflective article an have pursued an ongoing goal of developing launderably-durable retroreflective articles so that persons wearing retroreflective clothing can continue to be conspicuously visible after the clothing has been worn and cleaned many times.
In U.S. Pat. No. 4,763,985 to Bingham, a launderable retroreflective article is disclosed that comprises a layer of transparent microspheres, a specular reflective layer optically connected to each microsphere, and a binder layer into which the microspheres are partially embedded. Resins disclosed as being suitable for use as binder layers include polyurethane, polyesters, polyvinyl acetate, polyvinyl chloride, acrylics, or combinations thereof. The specular reflective layers are composed of two succeeding layers of dielectric material.
In U.S. Pat. No. 5,200,262 to Li, a launderable retroreflective article is disclosed that comprises a monolayer of metal-coated microspheres partially embedded in and partially protruding from a binder layer that comprises a flexible polymer having hydrogen functionalities and one or more isocyanate-functional silane coupling agents. The disclosed flexible polymers that possess hydrogen functionalities are crosslinked, flexible urethane-based polymers such as isocyanate-cured polymers or one or two component polyurethanes and polyols. This retroreflective article provides good laundering durability: it can withstand industrial laundering conditions, which involve wash temperatures as high as 40.degree. to 90.degree. C. (105.degree. to 190.degree. F.) and pH values of 10 to 12.5.
In U.S. Pat. No. 5,283,101 to Li, a launderable retroreflective article is disclosed that comprises a binder layer formed from an electron-beam curable polymer and typically one or more crosslinkers and silane coupling agents. The electron-beam curable polymers include chlorosulfonated polyethylenes, ethylene copolymers comprising at least about 70 weight percent of polyethylene such as ethylene/vinyl acetate, ethylene/acrylate, and ethylene/acrylic acid, and poly(ethylene-co-propylene-co-diene) polymers. Glass microspheres are embedded in the cured binder layer, and a specular reflective metal layer is disposed on the embedded portions thereof. This retroreflective article also has been shown to be durable under industrial laundering conditions.